Ultrasonic Flow Meter
STATE-OF-THE-ART VERSATILE FLOW METERS - Ultrasonic Flow Meter
The device consists of a pipe with transducers that emit and receive ultrasound signals into the flow of the medium.
The transducers used are either integrated transducers which consists of wetted transducers that come in contact with the fluid to be measured or Clamp-on transducers, which consists of transducers that are fixed externally to the pipeline.
Based on the measurement principle used, an ultrasonic flow meter can be distinguished into two types:
- Doppler flow meter
- Transit time flow meter
1. Doppler flow meter
This type of flow meter consists two piezoelectric transducers (either integrated or clamp-on transducers) fitted at a specific distance “L” from each other. An ultrasonic signal is transmitted by one transducer at a particular frequency and deflected by acoustic reflectors (i.e. air bubbles or solid particles) suspended in the flow medium.
This signal is then received by the second transducer with a frequency shift. This difference in frequencies is known as a Doppler frequency shift and is directly proportional to the velocity of the flow medium. Since we know the diameter of the pipe and the velocity of the medium, we can now calculate the volumetric flow rate from these parameters.
2. Transit time flow meter
This flow meter uses the difference in time that an ultrasound signal takes to travel the distance between transducers at a specific distance from each other to measure the volumetric flow rate of the medium. The signal first travels downstream (i.e. in the direction of the flow) and then upstream (i.e. against the direction of the flow).
The frequency of the transmitted signal is equal to the resonant frequency of the transducer. Since the signal travels downstream faster than upstream, there is a time difference called Time of Flight (TOF) or Transit Time and it is measured using a stop watch.
For example:
Consider two transducers A and B. The time taken by a signal to travel from A to B is TOFAB and that taken from B to A, TOFBA, is measured using a stop watch. The time difference is calculated as follows:
ΔT = TOFBA − TOFAB
In a no-flow state, ΔT is 0.
The flow velocity of the medium is directly proportional to the transit time. Since we already know cross-sectional area of the pipeline, we can calculate the volumetric flow rate using the transit time and the flow velocity.
Single path or multipath ultrasonic flow meters
Ultrasonic flow meters are further distinguished by the number of paths that an ultrasound signal takes from one transducer to the other. In flow meters with two transducers, the signal follows one path, from the transmitting transducer to the receiving transducer. They are called as Single-path flow meters and are ideally suited for pipes with small to medium diameters. Some ultrasonic flow meters use four transducers, in which case there are two signals which follow two paths. These are called as Dual-path flow meters. Similarly, ultrasonic flow meters which use multiple transducers are called multipath flow meters. They are most suitable for use in pipes of large diameters.